The flavonoid Quercetin (3,3',4',5,7-pentahydroxyflavone) has been shown to inhibit the activity of a variety of enzymes including the calcium- and phospholipid dependent protein kinase (protein kinase C) in vivo and in vitro. Furthermore, it synergistically enhances the antiproliferative activity of cis-diaminedichloroplatinum II (cis-DDP) both in vitro and in vivo and therefore is of interest as a promising therapeutic agent for use in the chemotherapy of human tumours. However, Phase I clinical trials have proved problematic owing to the limited solubility of Quercetin in pharmaceutically acceptable solvents, and this characteristic has prevented its further clinical development.